Among lysolecithins of a plant origin, the soybean lysolecithin is understood to mean a 2-monoacylglycerophospholipid or 1-monoacylglycerophospholipid being produced through modification by allowing phospholipase A.sub.1 or A.sub.2 as a hydrolysis enzyme to act on water-treated soybean lecithin (1,2-diacylglycerophospholipid) to partially hydrolyze the fatty acid ester moieties of such phospholipid. As compared with normal soybean lecithin, such soybean lysolecithin can offer the characteristic features, such as (1) enhanced o/w emulsifying property, (2) increased emulsion stability retained under acid conditions and in the coexistence of salts, (3) improved effects developed in capability to bind to proteins and starches, and (4) excellent mold-releasing or pan-releasing property, and consequently, the demand for soybean lysolecithin is growing in recent years.
Soybean lecithin, which is used as a starting material for such soybean lysolecithin, is ordinarily produced in the form of paste-formed lecithin being composed of 60 to 65 weight % of phospholipid, 35 to 40 weight % of neutral oil and slight amounts of free fatty acids and sterols by filtering raw soybean oil, adding for stirring 2 to 3 weight % of water to soybean oil as warmed at 60 to 80.degree. C., and subjecting a settled precipitate of a hydrated gum-like material (hydrated lecithin) to a degumming step of conducting separation by centrifugation, followed by heating for drying of the separated hydrated gum-like material under reduced pressure. And soybean lysolecithin is normally produced by adding water to such soybean lecithin (paste-formed lecithin) again, allowing phospholipase A.sub.1 or A.sub.2 to act on the mixture to hydrolyze the desired fatty acid ester moiety of each phospholipid under warming, heating the reaction mixture after conclusion of the reaction to thereby deactivate the enzyme, and then conducting heating and drying under reduced pressure, followed by filtration to remove the enzyme.
According as the demand for lysolecithins of a plant origin represented by such soybean lysolecithin has been increasing, however, the color phase, flavor and smell of the conventional lysolecithins are posing problems, being regarded as a matter of concern to the consumers. In order to alleviate or solve such problems, there has been developed the defatted lysolecithin which is manufactured by defatting a conventional lysolecithin by means of the extraction procedure with acetone to remove free fatty acids liberated during the enzymatic reaction and neutral oil, sterols, etc. derived from the starting material, with the result that the demand for the same is rapidly growing, but even such defatted product fails to solve the above-mentioned problems satisfactorily.
Thus, the discoloration and flavor deterioration of lysolecithin are in the first place brought about by heating the starting-material lecithin in the heating/drying step as described previously. In summary, lecithin, which is not heat-resistant and is easily susceptible to discoloration upon heating, is inevitably accompanied with discoloration in the concentration and drying step for the hydrated gum-like material, and tends to exhibit deteriorated flavor as well. Lysolecithin likewise is least heat-resistant, while on the other hand it has to be subjected to the enzyme deactivation step and also the second heating/drying step, and consequently, its discoloration and flavor deterioration inevitably constitute the problems being left unsolved. As a counter-measure for such defects, it may be conceivable to suppress the discoloration and flavor deterioration of lysolecithin by increasing the degree of depressurization in the drying step to thereby allow water evaporation at lowered temperatures. However this has been proven to produce the desired effect only to a limited extent, while it requires the filtration step to remove the enzyme, making the manufacturing process more complicated.